This invention relates to a load indicating fastener which is adapted to indicate loads to which it is subjected in use.
It is desirable that applied fasteners should be properly tightened to designed load levels in order to ensure that secure joints are achieved by the use of the fasteners. Torque wrenches are commonly used for tightening fasteners such as bolts to pre-determined loads. They measure applied effort and are subject to friction so that the fastener loads they produce may not be as accurate as they might be. Fasteners have been proposed which indicate when desired loads have been applied to items but such fasteners have tended to be expensive and/or complicated to install.
One form of load indicating fastener is known from French Patent No. 1,477,006 in which a graduated wire or strip is fixed at one end in an internal bore of a stud. Elongation of the stud under tensile stress is measurable against the fixed scale of the strip to provide an indication of applied load. In this arrangement it is difficult to accurately read the small elongation against the scale.
Another form of fastener is disclosed in UK Patent Application GB 2,212,284 in which a stem fixed at one end to the shank of a bolt carries an electrical contact at its other end. Extension of the shank under an applied load draws the electrical contact towards an axially facing surface of the bolt. The variation in capacitance between the electrical contact and the surface may be used to provide an indication of the bolts extension and, hence, the applied load. Alternatively, completion of an electrical circuit between the electrical contact and the surface may indicate that a predetermined load has been applied. This system has been found to be complicated and expensive to implement with the required degree of accuracy. In addition, it does not provide a visual indication of applied load.